Coaxial-type filter



Sept. 129 E95 J. s. FOSTER, JR

coAxIAL-TYPE FILTER Filed April 2, 1946 FIG.I

FIG2

FIGB

/NVE/VTOH. JOHN S. FOSTER JR.

A TTR/VEY Patented Sept. 12, 1950 UNITED y'ST'A'TIEIS PATENT OFFICE CAXIAL-TYPE y'FILTER .iohn S. Foster, Jr., Montreal, Quebec, Canada,

assigner to United States of America as represented by the Secretary of War v Application April 2, 1946,:Serial'No. 659,107

This invention relates .to radio frequencyl- `ters of the coaxial type andmore particularly to simply land accurately, and are capable of being T easily replaced. v

It is still another object of the invention to provide a microwave lter having band pass characteristics comprising ,a Aplurality of removable filter sections.

For a better understanding of the invention together with other Aand further objects thereof, reference is had to .the following description, taken in connection with the accompanying drawing, in which: l

Fig. 1 is a longitudinal sectional View ofthe lilter according to one embodiment of the present invention; v

Fig. 2 is 'a longitudinal view of an individual unit of the iilter of Fig. 1; and

Fig. 3 is a schematic diagram showing the equivalent circuit of the lter of Fig. l.

In general, the iilter Ill according to this 'invention, and shown in Fig. 1,' is similar to the conventional coaxial type v:iilter in'that it utilizes concentric capacitors a's 'the capacitor elements and sections of conductor line as the inductance elements.l As shown, filter Il) generally comprises a plurality of internal sections Il and two end sections I2, one at each end. Each internal section II (Fig. 2) comprises a short section of thin walled insulating material I3 generally in the form of a hollow cylinder. Two spaced disk .members or short cylindrical members I 4 of electrically conductive material are connected at their centers by a length of fine wire I5. The disk members I4 are `positioned with their planes substantially perpendicular to the longitudinal axis of i'ilter IB and are so positioned that their circumferential surfaces t snugly within the inner surface o'f the cylinder of vinsulating'material I3. If desired the inner surface of cylinder I3 may be provided with two shoulders I3 against which disks I4 may bear thereby being maintained in proper spaced relation. The wire I5 is stretched taut between the two disk members I4 5 claims. (cl. 17e-*44) and then soldered to form a secure connection to the 'center 4portions of disks I4. The wire 'I5 also `serves 'to prevent displacement of the disks 'I4 thereby 'holding the components of the unit or section Il together.

Each 'of the end sections I2 consists of a center conductor I6 .iirmly mounted centrally of a block "Il of insulating dielectricma'terial and of generally cylindrical cross-section. The free or outer 'end |25 of the center conductor It extends beyond the :block 'I l. Conductor i 6 may be of hollow or resilient 'construction 'adapted to receive the center conductor of a coaxial transmission line to which vthe l'ter maybe attached. Fitted trans- The 'internal sections II and end sections I2 are Jadapted to be received snugly within metallic 'tube' 2'I which forms the outer conductor of iilter 0.

In assembling lter I0, the individual sections 'as shown in Fig. 2 are tted together so that the disk members HI 'of adjacent sections II are 'spaced slightly apart. The vend sections are then placed 'so Vthat the disk members It and I9 are slightly spaced apart axially of the Iilter 'It and the whole is inserted in metallic tube 2! or alterfn'atively the sections I I and i2 may be inserted in proper sequence into tube 2l. If Vdesired and to 'facilitate connection to a coaxial transmission line in a circuit of a communication or radio object locating system, an R.-F. connector 22 may be secured to the free ends of end sections l2. R.F. connectors 22 may comprise generally a hollow cylindrical member having an inner end portion T23. adapted to iii-,snugly Within the end of metallic tube lZI and -having a bore 24 therevthrough to accommodate the end portion I6 of center conductor it and the inner conductor of thetransmissionline to lbe connected. The cuter circumferential :portion of the .R.F. connector 22 may, if desired, be provided with 'screw threads for engagement with the end portions of the coaxial transmission line to which the lter is connected.

As thus assembled, it will be seen that filter IU tube 2I, while series capacitance exists between the closely adjacent disks lil and between the disks I4 and I9. Thus, the filter I0 has the characteristics shown in the equivalent circuit of Fig. 3 where like numerals indicate like parts. While there have been shown only two individual units II, it will be obvious that filter IU may comprise any number of individual sections II. In general, the more individual units .'provided, the more the accentuation of the pass band may be achieved.

The design and dimensions of the various components of lter I as described may be determined according to any desired nlter design method, for example, according to the method as de scribed in the co-pending application of Seymour B. Cohn, Serial No. 658,939, entitled Filter, filed April 2, 1946. In the design of microwave band pass filters such as lter ill of the present invention, the use oi a high impedance section of line in the neighborhood of 200 to 300 ohms is often required. The mechanical construction as described according to this invention permits a line characteristic impedance of 200 to 500 ohms with satisfactory results.

It will be understood that a filter constructed according to the invention as hereinbefore described is adapted to be very simply and accurately constructed and the units easily replaced. The components of such a nlter are unique in that few materials are used and the machine operations and assembly are very simple, necessitating only lathe and soldering operations, It will also be understood that the several components of the filter I0 may be accurately held inposition without necessitating the use of close tolerances and laborious assemblies which have been heretofore necessary with filters constructed with methods heretofore known.

While there has been described what is at `present considered the preferred embodiment of the invention, it will be obvious to those skilled in the art that various changes and modifications may be made therein without departing from the invention.

What is claimed is:

l. A band-pass ilter of the coaxial type for use within the microwave range of frequenciesof electromagnetic energy comprising, in combination, a metallic tube forming the outer conductor, a plurality of removable filter sections slidably mounted in said tube, said lter sections comprising a plurality of internal sections arranged longitudinally of said tube and two end sections, each of said internal sections comprising a hollow cylinder of insulating dielectric material adapted to slide snugly into said tube, two short circular disk members of electrically conductive material mounted concentrically of said tube and cylinder with their circumferences snugly tting within opposite ends of said cylinder and a ne 4, wire secured to, and electrically connecting, the centers of said disks, each of said end sections comprising a cylindrical block of insulating dielectric material adapted to fit within said tube, one end of said block being shaped as a hollow cylinder, its opposite end having an axial bore therethrough, a solid conductor located snugly in said bore, a disk member of electrically conductive material adapted to fit snugly and concentrically within said hollow cylindrical end portion and a ne wire secured to and electrically connecting the centers of the inner end of said conductor and of said last-mentioned disk Vmembers, the disk members of adjacent-sections, When in assembled position within said tube being slightly spaced apart to afford series capacitance elements, shunt capacitance being afforded between the circumferences of said disk members and said tube, and series inductances being afforded by said conductors and said wires, and meansv for connecting the ends of said filter to lengths of coaxial transmission line in a circuit.

2,. A iilter as claimed in claim 1 wherein said wires are stretched taut and soldered to the center portions of said disks and said conductors thereby being eiective to maintain said disks in position and thus the components of each of said sections together as a unit.

3. A band-pass filter cf the coaxial type for use within the microwave range of frequencies of electromagnetic energy comprising a metallic tube forming the outer conductor, a, plurality of removable filter sections slidably mounted in said tube, each of said filter sections comprising spaced concentric capacitor elements, an inductance element comprising a center conductor of fine Wire electrically connecting said capacitor elements, and a hollow dielectric cylinder within which said capacitance and inductance elements are mounted and which is yslidably receivable in said tube.

4. A filter as recited in claim 1, wherein the number 0f said removable iilter sections determines the pass band characteristics of the iilter.

5. A band pass lter of the coaxial typerfor use within the microwave range of frequencies comprising a tubular member of electrically conductive material forming the outer conductor and a plurality of removable, unitary, and self-contained filter sections forming the inner conductor, said sections slidably mounted in said tubular member, each of said sections consisting of a hollow -dielectric cylinder dimensioned to t snugly and slidably in said tubular member and havmg an inductance element and capaitance elements secured thereto and wholly mounted therein, said inductanlce element being 'electrically connected to said capacitance elements, and said capacitance elements being capacitively related to said tubular member.

JOI-IN S. FOSTER, Jn."

4nl@ of lthis. patent:

4 UNITED STATES PATENTS Number Name Date,` y 1,905,353 Potter Apr. 25, 1933 2,183,123 Mason Dec. 12, 1939 2,274,346 Roosenstein Feb. 24, 1942 

